It sounds like maybe the gift-givers had forgotten to checkout using your registry. At least, that’s what I would assume would be the reasoning behind the duplicate purchases.

As far as their return system, unfortunately I believe the way they do it is the way most chain stores have to if you don’t have a receipt. If I were you, I’d have your DH return the rest of the items until he reaches his limit as well. It stinks, but I don’t blame the company for that policy- It’s all on your guests to make sure they include a gift receipt.